Caspar, once testosterone is started it is irreversible, in that the ovarian function will be affected, some more than others which is why the run up to having T therapy is much longer than for M>F as if the feminine hormones (oestrogen) are stopped the natural testosterone levels are quite quickly restored. Full fertilty is not guaranteed though.
(this is based on advice from Nottingham gender clinc)
I am sure there will be extensive psychological testing and discussion, but it might be worth flagging up to his consultant if it isn't already as they have to have an adult with them I believe if under 16.
There is a common misconception that in the UK hormones and life altering surgical procedures are handed out to anyone with dysphoria, but that is not the case. Many hoops have to be jumped through to assure the specialists that there is no underlying history of anything else Mental Health wise that needs addressing.
The key is getting your boy lots of counselling, encouraging discussion about all the possibilities, from staying physically a girl but presenting as a boy, dressing as one, right up to full hormones initially & then surgery at a (much much) later date. Also including a complete about turn later if he decides that's what he wants.
Some young teens are so confused about their feelings and the media is full of absolutely every possible life choice at the moment it's mind boggling. However keep up the lines of communication and loving support (even if you don't understand or agree with it!) is the key.